1. Field of the Invention
The invention is related to low-power mixed signal electronic circuits and in particular to power reduction for data conversion devices.
2. Description of the Related Art
Electronics systems benefit in many ways from reduced power consumption: lower operating costs, greater reliability, and the possible elimination of cooling elements such as fans to name just a few.
Reduced power consumption is particularly critical in battery-powered electronics systems. Not only does a battery-powered system benefit in all the ways just listed, reduced power consumption implies greater operating life between charges for a given battery size or, equivalently, a smaller battery may be used to obtain the same operating life between charges. The bulk and weight of a portable electronics system are severely constrained by the enduser's willingness to carry the system around. One example is the lack of acceptance of "luggable" portable computers which were available in the mid- to late- nineteen eighties, contrasted with the much greater popularity of lightweight "notebook" computers.
Similarly, extended operating life and small, lightweight packages contribute to the increased popularity of a wide variety of portable electronic processing devices such as cellular telephones, hand-held computers, data loggers, etc. Reduced power consumption plays an important role in reducing the size and weight and thereby increasing the acceptance of such systems.
Since low-power operation is especially desirable, many techniques have been employed to reduce a system's power consumption. One approach is to "power cycle" the electronic system, i.e., turn the system's power on only when necessary to perform critical operations. Because no power is consumed during the "powered down" periods, the system's average power consumption is reduced.
One problem with this approach to power conservation is that critical data, register values, instructions, etc., must either be stored within the circuit in a nonvolatile storage medium, such as erasable electrically programmable read only memory (EEPROM) or battery-backed static random access memory ("shadow RAM"), or it must be stored outside the system and reloaded when power is restored. Nonvolatile storage typically adds expense, weight and space to the circuit requirements.
Another approach to power reduction takes advantage of the widespread migration from TTL to CMOS integrated circuit technologies. Unlike TTL logic gates, CMOS gates dissipate very little power when they are in one logic state or the other; they dissipate power only when switching from one state to the other. Therefore, to reduce power consumption in CMOS circuits, switching speeds are reduced as much as design constraints will allow. An extreme form of this approach is evidenced by CMOS circuits such as microprocessors which feature "sleep" modes. In sleep mode, the system clock is shut off for whole sections of circuitry; no switching takes place and power consumption is reduced to almost nothing in that section of circuitry.
One advantage of the "sleep mode" approach over that of power cycling is that, because the power remains applied to the circuit, non-volatile storage is not required. As long as power is applied to the circuit, data, register values and instructions will remain intact. For a discussion of low-power design techniques see Paul Horowitz, Winfield Hill, The Art of Electronics, Second Edition, Cambridge University Press, New York, 1991 at pages 917-985 (sleep mode operation is discussed on page 975 and power switching is described on page 938).
Unfortunately, some circuits are not amenable to either power cycling or sleep mode operation. For example, an R2R digital to analog converter (DAC) switchably connects output and reference circuit sections through an resistor array. A voltage output DAC connects each input node of an R2R resistor ladder through analog switches to one of two voltage references; a current output DAC basically reverses the current flow, but both the voltage output and current output DACs would continue to dissipate power through their reference/resistor ladder circuitry even if their digital circuitry were "sleeping", i.e. not changing states. The DAC8800 is an example of a prior art voltage output DAC, the AD7541 is an example of a prior art current output DAC; both are available from Analog Devices, Inc., Norwood, Mass.
The power cycling approach to power conservation meets with its own problems when applied to R2R DACs. The DACs typically include an interface section through which data is loaded into the registers which control the DAC's switch configuration. As noted above, the registers must either be preserved or reloaded when power is restored after a "power down" cycle. Reloading the registers can be a time-consuming operation, especially if there are several registers and if the interface is a serial interface. Preserving the registers through the use of nonvolatile storage is often impractical for the reasons noted above.
There is thus a need for a DAC which preserves register values within its control section, and reduces the amount of power consumed by the its reference/output section, without removing power from the converter circuit and without employing storage for the register values.